In the application of liquid substances to a moving web of material, it is considered well known in the art to apply the liquid using a rotating transfer roller, and to directly apply the liquid uniformly onto the roller by means of a doctor blade assembly. The doctor blade assembly generally includes a reservoir chamber extending the length of the transfer roller and in contact with the circumferential surface thereof, and a pair of doctor blades extending longitudinally on either side of the chamber. The doctor blades are angled obliquely toward the transfer roller surface, and serve both to seal the reservoir chamber to the roller and to form a uniform film of liquid on the roller transfer surface. The assembly also must include some means to seal the reservoir chamber at the ends of the roller, so that the liquid is not flung from the roller into the surroundings, and so that the the liquid may be pumped through the reservoir during the transfer process. Such transfer systems are used in flexographic and gravure printing, adhesive applicators in the paper converting industry, coating applicators in many different industrial processes, and the like.
A persistent problem in prior art transfer systems is the sealing arrangement with the transfer roller. It may be appreciated that the transfer roller operates at high speed, on the order of 1000 linear feet per minute, and the end seals of the doctor blade assembly wear quickly. As the end seals wear, the applied liquid is flung from the transfer roller, causing a difficult and messy cleanup problem. Furthermore, the doctor blades themselves must be aligned with the roller with extreme precision, with tolerances to one thousandth of an inch. These two factors combine synergistically to reduce the productivity of the transfer system. That is, when the end seals or the doctor blades are too worn to be used further, the system must be shut down, the head must be removed, and the end seals replaced. Likewise, changing ink color in a printing press also requires removal and replacement of the doctor blade head. When the head is resecured to the transfer roller assembly, it must be carefully aligned to the transfer roller so that the liquid is once again uniformly applied to the roller and to the moving web of material. The steps of rebuilding or replacing the end seals and re-aligning the doctor blade head result in an unacceptable amount of down time for the transfer system. Clearly the prior art indicates the need for a system which reduces the time required to rebuild the end seals and re-align the head.
One approach known in the prior art provides a pair of stationary end seal members impinging on the ends of the transfer roller, so that the doctor blade head may form a seal with the stationary seals rather than with the moving surface of the transfer roller. This sealing arrangement requires sealing hardware to be mounted on the transfer roller and machine frames which is more cumbersome and costly. This system also does not incorporate a feature to easily remove and replace the doctor blade head.